While reciprocity failure can be beneficial, my experience is generally that increased contrast from reciprocity failure most often occurs when overall light levels are dim. In most such situations, the contrast is already quite high. In both the English cathedrals and the slit canyons, reciprocity failure saddled me with undesirable additional contrast in situations that were already excessively contrasty. (All films vary in their reciprocity failure characteristics; some, like Fuji Acros, have none up to two minutes of exposure.)
Another problem with long exposures is the fact that the gray meter does not calculate the film’s reduced response to low light levels. Within the negative’s normal response range you can determine your exposures as explained in this chapter and the previous one; but for long exposures (beyond one second) you must factor in additional time to compensate for reciprocity failure. Then you must alter contrast in negative development to compensate for the contrast increase during exposure.
Table 9-1. Exposure Table for Reciprocity Failure of Tri-X Pan Film (ASA 320)
Metered Exposure
Required Exposure
Contrast Increase
2 sec.
3 sec.
5 sec.
8 sec.
N + ½ *
10 sec.
18 sec.
15 sec.
30 sec.
20 sec.
45 sec.
N + 1 *
30 sec.
75 sec.
1 min.
3 min.
2 min.
7½ min.
N + 1½ *
4 min.
16 min.
10 min.
50 min.
N + 2 *
20 min.
2 hrs. 20 min.
N + 2½ *
30 min.
4 hrs.
N + 3 *
Table 9-2. Exposure Table for Reciprocity Failure of T-Max 100 Pan Film
Metered Exposure
Required Exposure
Contrast Increase
2 sec.
2½ sec.
5 sec.
7 sec.
10 sec.
15 sec.
N + ½ *
15 sec.
24 sec
20 sec.
35 sec.
30 sec.
50 sec.
1 min.
2 min.
N + 1 *
2 min.
4½ min.
4 min.
10 min.
10 min.
28 min.
20 min.
65 min.
N + 1½ *
30 min.
1 hr. 50 min.
In both tables (Table 9-1 and Table 9-2), the asterisk (*) indicates the approximate amount of contrast increase due to reciprocity failure and extended exposure. Thus, if you use Tri-X film and have a metered reading of 30 seconds, the table indicates that you must expose the negative for 75 seconds; but normal development of that negative will increase the contrast as if you had N+1 development (Figure 9-6–Table 9-4), so you must give the negative an N–1 to compensate for the contrast increase during the long exposure.
The reciprocity failure tables show the approximate exposure increase needed for extended exposures for both Tri-X and T-Max 100 films. The Tri-X table is my own personal table derived from years of practical experience (identical to Minor White’s own determination 40 years earlier). Kodak’s table for Tri-X is wildly incorrect. The T-Max table is based upon Kodak’s numbers, which are correct but incomplete.
It’s easy to interpolate between the stated times. The following example for Tri-X illustrates the proper method for using the table with the zone system. Suppose you want to place an object in Zone 7, and your gray meter gives you a five-second exposure reading. Of course, the reading places the object in Zone 5 (as always!), so first you double the exposure to 10 seconds in order to place it in Zone 6. Then you double the shutter speed to 20 seconds to place it in Zone 7. By consulting the reciprocity failure table, you determine that 20-second metering requires a 45-second exposure for Tri-X film. With T-Max 100 film, the metered 20-second reading requires a 33-second exposure.
If you’re using color film, reciprocity failure can be fascinating. Color film is made up of three separate emulsions, one for each primary color. Each has its own rate of reciprocity failure, so the color balance of the film tends to shift as exposures progressively lengthen. Colors that do not exist in reality may show up during long exposures, and some colors may be greatly enhanced, while others may be lost. Anything can happen. The effect can be wonderful or awful, but it’s usually surprising. It also tends to vary with each emulsion batch, so it’s never consistent from roll to roll or sheet to sheet. Reciprocity failure could open up areas of wonderful creativity for those interested in pursuing its eccentricities.
Examples of Decreasing and Increasing Contrast
As a first example, let’s return to the sunlit snowfield of Chapter 8. As before, you see the modulations of the snow, its hills and valleys. You want to photograph it so that those delicate tones are visible in the print. If nothing but snow is in the scene, it’s easy. All you have to do is decide where to place the snow on the scale and shoot. My choice would be an average placement of Zone 7½ or 8, which would yield a range of light tones from pure white to light gray and would show the modulations.
But suppose a large, dark boulder sits in the center of the field, oriented in such a manner that it’s shaded. Let’s agree that the rock should be about a Zone 3 tonality, and that the snow should still be placed at about Zone 7½ or 8 on the average. Suppose the gray meter gives a reading on the snow of f/16 @
The difference is 5 stops, or 5 zones. No matter how you expose the negative, there will be a 5-zone spread between the snow and rock, which cannot be changed in the exposure. It may not be wise to expose at either the metered reading on the snow or the rock; if you choose the former, the snow would come out at Zone 5 and the rock at Zone 0, while if you choose the latter, the rock would be exposed at Zone 5 and the snow at Zone 10. But you know how to place the rock at Zone 3: close down stops from the gray meter’s reading on the rock. With the rock in Zone 3, the snow is automatically placed at Zone 8 on the exposure scale, and normal development brings the density of each to the desired level. (Further on I explain why I actually prefer the shadow placement to be in Zone 4 rather than Zone 3.)
Suppose, however, that the meter reading on the rock is f/4 @